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Sightings abound of a four-legged, hairless, fanged monster that kills and sucks the blood, and sometimes milk, from livestock in the United States and Latin America. Its name chupacabra literally means "goat sucker."

There is, in fact, a real monster behind the sightings, but it has eight legs, measures at most 0.02 inches long and burrows into skin, rather than sucking blood. Its name: Sarcoptes scabiei,the mite that causes scabies in humans … and coyotes.

The chupacabras themselves are actually coyotes with severe infections by these mites, called sarcoptic mange, according to Barry OConnor, an evolutionary biologist at the University of Michigan.

Infections in humans are usually mild, causing patches of itchy skin. Over our long evolutionary history with the mite, we have gained the ability to fight off the infections. Domesticated dogs and other animals have less experience with the mite than we do, and for them sarcoptic mange infections can be severe. But the prospects are much worse for their wild relatives who have no experience with the disease, and it often kills them, he said.

"To me, the most interesting aspect of this whole system is the fact we are talking about a human parasite that has moved from us onto other animals, as opposed to all the things that have gone in the other direction," OConnor told LiveScience.

Reports of chupacabras began in Puerto Rico in 1995, where the creature was said to stand on two legs and have spines on its back. Sightings spread, with reports of the creature being spotted elsewhere, particularly Mexico, where it was described as a four-legged animal, but still hairless and ugly. People began taking pictures, which, according to OConnor, revealed the truth.

"The photos clearly show coyotes or dogs with very severe sarcoptic mange," he said.

The mites burrow into the animals' skin causing them to lose their hair and provoking an immune response that causes their skin to become thickened and ugly. Their faces swell, and their canine teeth become more prominent, resembling fangs. Weakened by the infection, they may be more prone to attack livestock, rather than their usual prey, such as rabbits, he said.

There is evidence for other sources as well. A strange, hairless carcass found on a golf course in Texas was dubbed a chupacabra. But a wildlife biologist examination revealed a close resemblance to a raccoon. The creature also apparently suffered from several diseases that can cause hair loss.